1. FIELD OF THE INVENTION
The present invention relates to an improvement in a variable speed electric hoist having an invertor unit as a variable speed controller for a lifting motor.
2. DESCRIPTION OF THE PRIOR ART
As an example of the variable speed electric hoist of the kind as described above, conventionally, there has been proposed an electric hoist such as disclosed in Japanese Patent Unexamined Publication No. 58-183594. In this variable speed hoist, the output frequency of an invertor unit is increased/decreased by a speed setter. If the output frequency is selected to be extremely low in setting the speed of a lifting motor, it is possible that a hung load will fall down during the lifting operation, because the lifting motor is a basket-type induction motor and its torque becomes extremely low particularly when run at a low speed.
Further, the speed of rotation of the lifting motor becomes lower than its synchronous speed by a value corresponding to a slip in a lifting operation mode because power running is performed in this period, while the speed of rotation of the lifting motor becomes higher than its synchronous speed by a value corresponding to a slip in a lowering operation mode because regenerative running is performed in this period. Accordingly, a considerably large difference occurs in the speed of rotation between the lifting and lowering operation modes in spite of the same speed setting, and the difference becomes larger particularly during low speed operation.
In a conventional variable speed electric hoist such as described above, there has been a disadvantage in that a hung load often falls down when the output frequency is made extremely low, or a large speed difference sometimes occurs between the lifting and lowering operation modes in low speed running. As a counter measure to this disadvantage, Japanese Patent Unexamined Publication No. 61-101397 discloses a variable speed electric hoist which is capable of preventing a hung load from falling down during low speed operation and which produces a small speed difference between lifting and lowering operation modes.
However, this variable speed electric hoist has a problem in that the hoist is arranged so that a brake is turned off during a first condition on the basis of a signal produced by an invertor while the invertor is generating a three-phase AC current, and there is therefore a possibility that a hung load may fall down if a signal for turning-off the brake is produced from the invertor in spite of the second condition that the three-phase AC current is not generated by the invertor in case of a failure of the invertor.